How to stop unreasonable defensiveness

Hello people. I could do with some advice if people can give it. I have ADHD, I am in my late 30s and have been formally diagnosed for 20 months now. I take Ritalin as medication on slow release which helps immensely, when it is in my system.

I have increasingly been having arguments with my fiance. We have been together for 8 years now and engaged for 3. The arguments mostly start from practically nothing, she asks a question, and I interpret that question as though she is questioning if I did something correctly or queries why something did not happen as she expected. I immediately start feeling judged and respond in either a negative way or lie and start obfuscating. I seem to have lying down to a fine art and can spin quite a pretty tale; how ever my other half is extremly good at detecting this (8 years with me) and refuses to put up with me a) lying and b) trying to not accept responsibility for either something not going quite right or more usually for assuming she is accusing me by her questions. "Did you put the garbage out?" "Why did you leave the light off?" "What is that cardboard box doing in the study?"

These sort of questions are (mostly) answered sensibly by me when I am in a medicated time period, "No dear, I'll do it in 30 mins [I there and then set an alarm to remind me]." "I was just being lazy dear?" "I left it there to remind me to fill it with the paperwork this evening."

When I'm not on the medication or tired (or both) my responses are like, "Well it wasn't on my list of things to do this evening so I forgot!" "I though I could see ok in the dark for what I needed to do. It doesn't matter, why are you questioning my decision making?" "I'll move it ok! Right now dear [I stop doing something necessary to tidy away the box, and later forget to organise the paperwork]."

This causes arguments, often prolonged ones because until my cortisone and dopamine levels (I think) are raised enough by having the argument. I am *convinced* that I was actually accused of something in the questioning. My other half is quite a forthright woman and when she wants to accuse you of something there is no ambiguity about it (with me an other people) so please don't be thinking her questioning is some passive-aggressive thing

Some of the time my fiance notices what is happening and quells it by ignoring me and walking away from the argument (fairly soon after when the 'challenge' is removed I tend to concluded it's me who as the problem and I apologise) but other times she is so totally surprised that this doesn't happen. 

I hate these arguments, they are hurting our relationship badly, and increasing in frequency and severity. Counseling is untenably expensive for us at the moment (saving for wedding) so if anyone has ay ideas it would be much appreciated.I am primarily interested in stuff *I* can do as my other half has show great flexibility in modifying other aspects of her life (household org. planning, finance management, never complaining about forgetfulness, etc...), asking her to bend over this would be a bit insulting.